[sub-spm-optional-products] Add LCECryptoKit and LCFeatureControl as optional sub-SPM products

LCEssentials installs standalone; each sub target-depends on it so linking a
sub's product always pulls LCEssentials in too, without the consumer having to
declare it separately.

- LCECryptoKit: internalized from the remote LCECryptoKitBinary git dependency
  (embedded token URL removed) into a local binaryTarget vendoring
  Frameworks/LCECryptoKit.xcframework. LCECryptoKitManager moved out of
  LCEssentials core into its own LCECryptoKitManager target/product; the
  no-op fallback for when the binary wasn't linked is gone (breaking change
  for existing consumers, see decisions/2026-09-15-sub-spm-optional-products.md).
- LCFeatureControl: new product wrapping Atomenta's Feature Control API
  (flag evaluation with TTL cache + safe-degrade fallback to defaults,
  notifications inbox, batched exposure telemetry). 45 new tests.

Sources/LCFeatureControl/FeatureControlContext.swift

@@ -0,0 +1,30 @@
+import Foundation
+
+/// Who/what a flag evaluation is for — mirrors Atomenta's `context.subjectType` enum.
+public enum FeatureControlSubjectType: String, Codable, Sendable {
+    case user, customer, store, anonymous
+}
+
+/// Evaluation context sent as `context` in `POST /api/feature-control/evaluate`.
+public struct FeatureControlContext: Encodable, Sendable, Equatable {
+    public var subjectType: FeatureControlSubjectType
+    public var subjectId: String
+    public var storeId: String?
+    public var platform: String?
+    public var appVersion: String?
+    public var attributes: [String: String]?
+
+    public init(subjectType: FeatureControlSubjectType,
+                subjectId: String,
+                storeId: String? = nil,
+                platform: String? = nil,
+                appVersion: String? = nil,
+                attributes: [String: String]? = nil) {
+        self.subjectType = subjectType
+        self.subjectId = subjectId
+        self.storeId = storeId
+        self.platform = platform
+        self.appVersion = appVersion
+        self.attributes = attributes
+    }
+}